How to connect Blender to Arduino?
The way I did it was by writing serial information to the usb port from the arduino and reading it from inside blender with pyserial.

I did run this in the Blender's game engine, which may or may not be what you want. The Blender side looks something like this (code is not very clean, I just hacked this together quickly for fun, but hopefully to gives you some idea of how you could implement it):

from bge import logic
import serial
from mathutils import *
from math import *


def startup():
    ser = serial.Serial('/dev/ttyUSB0')

    scene = logic.getCurrentScene()
    object = scene.objects['Suzanne']
    print('startup')

    logic.globalDict['serial_bus'] = ser
    logic.globalDict['main_object'] = object
    logic.globalDict['light'] = scene.lights['Lamp']

def loop():
    ser = logic.globalDict['serial_bus']
    object = logic.globalDict['main_object']
    light = logic.globalDict['light']

    # this is where the data is actually read from the serial interface
    data = ser.readline().decode()
    pot, var, phot = data.partition(";")
    print("pot:", pot, "phot:", phot)

    object.localOrientation = Euler((-.31,0,radians(float(pot)))).to_matrix()
    light.energy = 10 - float(phot) * (10.0/800.0)


The Arduino side is just Serial.printing numbers separated by semicolons.

How exactly you want to animate objects in blender depends on your usecase. If it's for a real time demo, you probably want to use the BGE (or use another game engine entirely, such as godot). If you want to use it as a secondary input device in Blender's viewport (e.g. a sort of DIY 3D mouse), you'll probably want to look at writing an addon.